Starting the operation of a turbine engine is an essential but also a difficult stage, in particular in the case of relatively small engines. Most often, small turbine engines, e.g. auxiliary power units, are optimized to operate within a limited operational regime requiring airflow pressure and temperature that may be quite different from the ambient conditions existing when the engine is started.
In broad terms, a turbine engine operates by intake of a mass of external air that passes through an intake section disposed in the fore portion of the engine, compresses that air by passing it through a rotating compressor, which is positioned in a compressor section accommodated downstream of the air intake. The compression process increases the pressure and the temperature of the air.
The compressed air then flows downstream into a combustion section including a combustion chamber, in which fuel is atomized or vaporized and mixed with a portion of the incoming air, whereas the remaining air gradually enters the combustion chamber to dilute and cool the gases produced by the combustion process. Next, the fuel-air mixture is ignited, whereby the temperature increases, and the combustion gases then flow downstream as exhaust gases to a turbine section, and through a turbine, which is rotated thereby. Typically, the turbine may be rotationally coupled to the compressor by a common shaft. From the turbine the exhaust gases flow downstream through an exhaust section, where their energy is used for driving an energy-consuming unit, or for producing thrust.
The various portions of a turbine engine, including the intake section the compressor, the combustion chamber, the turbine and the exhaust section as well as various auxiliary equipment that were not mentioned hereinabove but are necessary for proper operation of the turbine engine, are integrated in the engine containing the portions and sections mentioned hereinabove.
The combustion chamber includes typically, but not necessarily, an annular flame tube consisting of two concentric liners (perforated walls) and a dome that are connected upstream. The inner liner surrounds the shaft and the outer liner is mostly disposed in concentricity with and in the interior of a combustion chamber housing. It is pointed out that the inner and the outer liners may also be of any other shape, generally conical or of another shape, and not necessarily concentric, but concentric cylindrical liners will be described hereinbelow as an example selected for the sake of simplicity only.
A turbine engine has to be started and brought into operational regime before it can function and provide the desired power. The starting process may be initiated when the air pressure in the combustion chamber(s) is much lower than the specific values for which the turbine engine is designed, because, for example, of the low rotational speed and low compression of the air. Therefore, the conditions in the combustion chamber(s) may not produce sufficient energy for accelerating the rotational speed of the engine to its operational regime. Should harsh ambient conditions be encountered, the starting process will become harder to achieve, and may be even impossible without additional provisions.
Various solutions have been proposed to bestow enhanced starting capability on an engine. U.S. Pat. No. 5,231,822 by Shekleton, referred to hereinbelow as Shekleton, recites a turbine engine having an enhanced starting capability achieved by controllably bleeding compressed air from the air flow path by means of valves and ducts. Thereby, Shekleton adds components and weight in the form of peripheral equipment in order to improve the starting process.
Most solutions include a combination of some kind of an external igniter (usually a pyrotechnic, spark-plug, glow-plug, pilot flame or other) and an external pressurized oxygen tank. The igniter provides heat into the combustion chamber for the purpose of igniting the fuel-air mixture, whereas the oxygen added to the combustion chamber enhances the combustion process, enabling a more reliable starting. U.S. Pat. No. 6,941,760 by Jones, referred to hereinbelow as Jones, discloses a starting system for expendable gas turbine engines having a pressurized oxygen tank (at some 5000-7000 psig). The oxygen flows into the turbine engine through two lines, one providing an impinging jet on the compressor blades, for rotation of the rotating assembly, and the other entering directly into the combustion chamber. The solution presented by Jones demands the installation in the engine of additional equipment such as tubes, valves and regulators, and further ancillary equipment, all of which are rather voluminous and heavy. Another disadvantage of this solution resides in the necessity to add equipment that operates only during the starting process, but remains on-board to become deadweight ballast after the starting procedure is completed.
Under favorable ambient conditions, a high-energy igniter may be sufficient for supplying the necessary addition of energy to the combustion chamber. Most high-energy igniters are pyrotechnic, normally disposed on the inner or outer side of the housing, outside of the combustion chamber, while the hot gases are discharged into the combustion chamber via a nozzle. U.S. Pat. No. 6,374,592 by Box et al., referred to herein below as Box et al., recites a turbine engine with an internal quantity of solid fuel. The patent describes a solid fuel element, with an igniter that are both disposed in the inner space of the combustion chamber of a turbojet engine. No available oxygen is discharged or may be used for enhancement of the combustion process of the fuel-air mixture under harsh conditions. Actually, Box et al. replaces the external pyrotechnic igniter, well-known in the art, by an internal one.
U.S. Pat. No. 4,161,102 to Jasas et al., referred to herein below as Jasas et al., recites a turbine engine starting system that comprises an auxiliary combustion chamber having a source of combustible fuel and oxidizer connected to the combustion chamber. A fluid passage is linked at one end to the auxiliary combustion chamber and at its other end is open to the turbine blades to rotatably drive the turbine engine. A portion of the hot exhaust gases from the auxiliary combustion chamber is preferably diverted and injected with supplemental oxygen into the main combustion chamber of the turbine engine. Jasas et al. thus require an auxiliary combustion chamber in addition to the conventional combustion chamber(s). Evidently, Jasas et al. requires the addition of weighty hardware dedicated to the starting process.
Therefore, it would be advantageous to provide a method for the implementation of a system and of a product designed for the enhancement and assurance of the starting procedure for turbine engines under various ambient conditions, but without requiring either the addition of any equipment remaining coupled to the turbine engine after completion of the starting procedure, or demanding the jettisoning of devices becoming superfluous after the starting procedure is concluded.